Wow, I am now a firm believer that when a child is really starting to pick up words and forming sentences, one should spend as much time as possible socializing with adult friends/family and older children!
We recently took D to New York and to LA, just for fun and to visit with friends and family but because she was exposed to all conversation, I swear her vocabulary has expanded by leaps and bounds! She literally will parrot almost everything she hears and then she sits and processes how the words are used, in what context.
One thing that D has been saying lately that I think is really neat is - after we go somewhere fun, like the zoo or the playground, she will say, "Was that fun??" And then I repeat it back to her and she responds, "Yeah!!"